Conventional contact exposure systems commonly retain original photographic master and copy sheets in contact registration against a glass platen during exposure through the glass platen. Substantially uniform force may be exerted on the photosensitive materials to retain them in registration against the platen by laying an impervious vacuum blanket over the sheets and evacuating air from the region between the blanket and platen. One difficulty encountered with conventional systems of this type is that residual pockets of air may not be evacuated from between the photographic sheets, resulting in the sheets containing air bubbles and creases that distort the reproduced image. Although several schemes using glass platens have been devised to ensure more accurate evacuation of air from within the region between the photographic sheets, these schemes still have the disadvantages associated with using a glass platen, namely, significant intervals of vacuum pumping that slow down the throughput rate of photographic reproductions and potentially longer than necessary exposures times.
Schemes using a vacuum contact plate instead of a glass platen are known. FIG. 1 illustrates the underside of a screen frame 6 used in one known vacuum contact scheme. The screen frame 6 includes a rigid outer frame 8 that supports a woven mesh or screen 10 thereon in planar relationship with one face of the frame 6. A photosensitive material, such as a conventional photographic emulsion that becomes impervious or insoluble after exposure to ultraviolet radiation, coats the screen 10 in a selected area 12 (preferably on the rear side that faces out or up above the face of the frame 8) within the perimeter of frame 8. The coated area 12 is sized to leave at least about a one inch intermediate border of clear or uncoated screen 14 on at least two sides of the screen 10 between the coated area 12 and the frame 8.
Referring now to the perspective view of FIG. 2 and the sectional view of FIG. 3, a known vacuum contact plate 18 includes an outer frame 20, a centrally-oriented screen support section 22 spaced inwardly from the outer frame 20, and a recessed area 24 located between the inner perimeter of the outer frame 20 and the screen support 22. The vacuum contact plate 18 receives a prepared screen frame of the type illustrated in FIG. 1 in inverted orientation with the frame 8 of the screen frame 6 disposed within the recessed area 24 and the screen 10 disposed over the central screen support 22. A master film 31 of sufficient size or a transparent cover sheet including the master film 31 is then placed above the screen 10, replacing the conventional glass platen. A vacuum port 26 communicates with the recessed area 24 to quickly evacuate air from the recessed area 24 and from space between the master film 31 and the screen 10. Vacuum contact plate schemes evacuate air from between photographic sheets at a significantly faster rate than glass platen schemes. The evacuation of air produces a pressure differential across the master film 31 and a resulting force which urges the master film 31 against the screen 10.
As shown in FIG. 4, a difficulty with the known vacuum plate 18 arises when the thickness of the frame 8 is less than the depth of the recessed area 24. In this case, pumping air out through the vacuum port 26 exerts a pull on the frame 8 if the frame is not shimmed up to a larger size. The pull creates stress on seam 34 and often results in separation between the screen 10 and the frame 8. Thus, although known vacuum plate schemes overcome many of the problems associated with glass platen schemes, they do not easily accommodate frames of various sizes, and a need exists for a vacuum contact plate system which provides more flexibility with respect to frame size.